Thermally insulated pipelines

ABSTRACT

A thermally insulated pipeline assembly comprises an inner pipe surrounded by and spaced from a jacket, which is in turn surrounded by and spaced from an outer casing. The annular space between the inner pipe and the jacket is substantially filled with thermal insulating material such as open cell polyurethane foam, which is preferably injected into the jacket and cured in situ. The space between the jacket and the outer casing may be filled with pressurized gas. The jacket may have apertures to allow the gas access to the annular insulated space. The arrangement provides improved thermal insulation, particularly when the casing is pressurized, by reducing thermal transfer by convection between the inner pipe and the outer casing. The arrangement is also applicable to pipeline bundles, with a plurality of inner pipes each having an associated jacket and insulation, all surrounded by a common casing.

TECHNICAL FIELD OF THE INVENTION

The invention relates to thermally insulated pipelines, particularly butnot exclusively subsea oil and/or gas pipelines, and to methods offorming such pipelines.

BACKGROUND OF THE INVENTION

In deep water offshore oil field developments, pipelines are installedto transport crude oil and gas from subsea well-heads to fixed platformsor floating storage facilities.

Crude oil contains many different chemical components and substances,from gases to semi-solid hydrocarbons, such as asphalt and paraffin wax,and frequently also water. Under the high underground pressures andtemperatures, the crude oil flows easily in a liquid or gaseous state.When the hot crude oil comes from the reservoir to the ocean floor andenters the pipeline for transportation to the platform, it gets intocontact with a cold sea water environment, which at larger depths (1000ft+305 m) has temperatures of about 40 degrees F. (4–5° C.). Under theseconditions the crude oil will cool down rapidly. When cooled down, waterand the semi-solid components and/or gases in the crude oil tend tosolidify forming hydrate and paraffin deposits on the pipeline wall.Consequently the pipeline cross-section is reduced and the flow capacityis diminished, adversely affecting the oil production. In extreme casescomplete stoppage of the pipeline may occur.

To prevent the build-up of paraffin and hydrates, pipeline and flowlinescan be insulated on the outside with thermal insulating materials, toreduce heat loss of the flowing crude oil, and to guarantee a requiredarrival temperature at the separating facility. Successful thermalinsulating materials used in offshore pipeline applications includepolyurethane foams, and epoxy and urethane based syntactic foams withglass microspheres.

One commonly used form of pipeline insulation consists of a“pipe-in-pipe” (PIP) structure, in which thermal insulation is appliedto the surface of a pipeline and the insulation is surrounded by anouter pipe or “casing pipe”. Generally, there is an annulus between theinterior surface of the casing pipe and the outer surface of the thermalinsulation. A number of insulated pipes (a “pipeline bundle”) may beenclosed in the same casing pipe. Commonly, the thermal insulationcomprises a plurality of C-section panels of insulating material securedto the outer surface of the pipeline. Flexible panels are also used,which can be deformed to wrap around the pipeline. It is also common topressurise the interior of the casing pipe using, for example, nitrogen.This improves the resistance of the casing pipe to collapse underhydrostatic pressure and thus allows the wall thickness of the casingpipe to be reduced. Where the thermal insulation comprises anopen-celled foam, the pressurising gas will also penetrate the foam.

In an insulated PIP assembly of this general type, one mechanism forheat transfer between the pipeline and the casing pipe is by convection.This is particularly so in the case of open-cell foam insulatingmaterials. The use of pressurised gas in a PIP system increasesconvection and hence increases heat transfer between the insulatedpipeline and the casing pipe. Where the thermal insulation comprisesC-section panels or other discrete panels or the like, gaps usuallyexist between adjacent panels, typically of the order of ⅛th inch(0.3175 cm). The present inventors have found that the presence of suchgaps seriously reduces the effectiveness of the thermal insulation, as aresult of convection when the casing pipe is pressurised. Thisconvective heat loss effect is negligible when the gas in the casing isat atmospheric pressure, but becomes increasingly significant withincreasing casing pressure. WO99/05447 discloses a deep sea insulatedpipe encased by an insulating core, the pipe having a protective outercasing. The insulation being made from microspheres contained in a resinbearing foam.

DISCLOSURE OF THE INVENTION

The invention is concerned generally with the insulation of subseapipelines.

The invention provides a pipeline assembly (particularly for carryingoil and/or gas from subsea accumulations of hydrocarbons long the seabedor from the seabed to the sea surface), comprising an inner pipe, ajacket surrounding and spaced from the inner pipe so as to define agenerally annular space between the inner pipe and the jacket, and anouter casing surrounding and spaced from the jacket, the space betweenthe jacket and the casing being capable of containing gas under pressureand in which the jacket includes apertures to allow the gas underpressure access to the generally annular insulated space between thepipe and the jacket said space being substantially filled withinsulating material.

In preferred embodiments, in which the space between the jacket(s) andthe casing is capable of containing gas under pressure.

In these embodiments, it is preferred that the jacket includes aperturesto allow the gas under pressure access to the generally annularinsulated space between the pipeline and the jacket.

It is preferred that the jacket is of high density polyethylene or ofother thermoplastic or metal.

It is also preferred that the inner pipe is supported within said jacketby first spacer elements disposed along the length of the inner pipe,and that the jacket is supported within said casing by further spacerelements disposed along the length of the jacket.

There may be at least two inner pipes surrounded by and spaced fromindividual jackets, the jackets being surrounded by and spaced from thecasing.

Preferably, the insulating material is open cell foam, such aspolyurethane foam, and suitably having a density in the range 1 to 15lb/cu ft, (16–240 kg/m³).

In accordance with another aspect of the invention, there is provided amethod for the production of a pipeline assembly in accordance with thefirst aspect, comprising the steps of arranging at least one inner pipewithin at least one jacket with a generally annular space therebetween,introducing insulating material into the annual space between the atleast one inner pipe and the at least one jacket, and arranging a casingto surround the at least one jacket with a space therebetween andpressurising the interior of the casing.

It is preferred that the thermally insulating material is introduced ina liquid state, and is allowed to solidify into an open cell foam withinthe annular space.

It is further preferred that the jacket is arranged to lie generallyhorizontally during introduction of the insulating material.

It is still further preferred that the jacket is arranged to lie at aslight slope during introduction of the insulating material, and thenarranged to lie horizontally during curing of that material.

The method preferably includes disposing first spacers along the lengthof the inner pipe whereby said inner pipe is supported within saidjacket.

The method preferably further includes disposing further spacer elementsalong the length of the jacket whereby said jacket is supported withinsaid casing.

The method preferably further includes the step of providing the atleast one jacket with small apertures to allow pressurised gas topenetrate the insulating material.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:—

FIG. 1 is a cross section of a first embodiment of a pipeline assemblyin accordance with the present invention;

FIG. 2 is a cross section of a second embodiment of a pipeline assemblyin accordance with the present invention;

FIG. 3 is a cross section of a detail of part of the pipeline assemblyof FIG. 1;

FIG. 4 is a longitudinal section of one end of a length of the pipelineassembly of FIG. 1; and

FIG. 5 is a longitudinal section of part of a length of the pipelineassembly of FIG. 1.

DESCRIPTION OF THE SPECIFIC EMBODIMENT

This description includes schemes for the manufacture of thermalinsulated single pipe lengths, typically of steel, and for themanufacture of thermal insulated multiple pipe lengths (bundles). Thesteel pipe is enclosed in a jacket using state of the art materials forthe jacket, such as steel, aluminum, high density polyethylene, or otherthermoplastic material. The jacket is enclosed in a casing. Theinsulation is suitably an open cell polyurethane foam and allows theannulus between jacket and casing to be pressurised with nitrogen, airor other gas to enhance the collapse resistance of the casing fromexternal pressure in deep water. The combination of pipe, insulation,jacket, pressurised gas and casing will be described as a pipelineassembly. This description also includes options for the commercialfabrication and installation of pipeline lengths.

Description of the Manufacture

As shown in FIGS. 1 and 2, the manufacture consists of one continuoussingle thermal insulated steel pipe 10A, or, multiple continuous singlethermal insulated steel pipe 10B bundled together. The single ormultiple pipes are encased in an outer steel pipe 12A/12B, hereafterreferred to as a casing. Each of the pipes 10A/10B is separatelycontained in a pipe jacket 11A/11B hereafter referred to as a jacket,which can for instance be manufactured of a high density polyethylene.The annulus 14A/14B formed by the pipe and the jacket is filled with anopen cell thermal insulation. The space 15A/15B inside the casing12A/12B which surrounds the pipe/jacket assembly(s) is filled with highpressure gas.

As an alternative to high density polyethelene, the jacket 11A/11B canbe manufactured of steel, aluminum, urethane or other suitable material.

Provision of a jacket to contain the thermal insulation around the pipeenables the thermal insulation to be formed without significant voids,by using the jacket(s) 11A/11B as a mould to form a continuous layer ofthermal insulation in situ on the pipe(s) 10A/10B. Injection of thethermal insulation as a liquid allows the thermal insulation to setwithin the jacket 12A/12B. This is particularly important when thecasing is pressurised. In these circumstances, the impact ofpressurisation on the overall heat transfer coefficient is significantlyreduced as there are no gaps in the insulation. The jacket may beprovided with small apertures 18 (FIG. 4) to allow the pressurised gasto penetrate the thermal insulation material so as to avoid any risk ofthe thermal insulation layer collapsing under pressure.

Fabrication of Thermal Insulated Pipe Lengths

As shown in FIGS. 3 and 4, one single steel pipe length 10A, typically40 to 80 ft (12 to 24 m) long, is inserted into a jacket length 11A,slightly shorter than the pipe length, to allow for a field weld duringthe assembly of the pipeline(s). Spacers 16 are attached to the pipe 10Ato maintain a constant position of the pipe relative to the jacket 11A.These spacers 16 can be fabricated of high density polyethylene,urethane or polypropylene material and can be mounted on slidingrunners, to minimise frictional loads during the insertion.

The pipe and jacket lengths are closed off at both ends with concentricshaped end closures 17. The end closures may also be manufactured ofhigh density polyethylene, urethane or polypropylene material.

An open cell polyurethane thermal insulation material of light unitweight (2 to 5 lb/cu ft 32 to 80 kg/m³) is injected into the annularspace between the steel pipe 10A and the jacket 11A. Optionally, thejacket has holes 18 to allow nitrogen into the jacket. This may beundesirable with certain insulation materials such as C-panels, asdescribed above, where it is desirable to isolate the thermal insulationfrom the pressurised gas inside the jacket. In addition to polyurethaneother types of insulation such as syntactic foam may be used.

When injecting the thermal insulation, the jacket is preferably set at aslope to allow for a better flow of the material. The thermal insulationwill develop heat during the chemical reaction of its components,therefore partial pours may be required. After the thermal insulationhas been poured and the length is completely filled, then the jacket isset back in a horizontal position for cure of the thermal insulation. Asa rule 90% of cure should be attained in 24 hours.

The required thickness of thermal insulation is governed by the OD ofthe pipe 10A and the selected ID of the jacket 11A.

The steel pipe may be coated with an anti-corrosion coating, for examplefusion bonded epoxy.

The pre-fabricated lengths of thermal insulated pipe can then betransported to the pipeline fabrication site.

Assembly of Pipeline(s) and Casing Lengths

The pre-fabricated thermal insulated pipe lengths are assembled intocontinuous pipeline(s) length(s). The pipeline fabrication site istypically on the shore, suitably a beach, with access to the ocean.

At the field joints between pipe lengths, half shells of jacket will besecured to the existing jacket, for example, forming an annular spacearound the pipe. Thermal insulation material, such as polyethylene opencell foam (5 to 20 lb/cu ft 80 to 320 kg/m³) or a syntactic foam (epoxyor urethane base with glass spheres), is inserted into this space. Thepurpose of the foam is to act as a thermal insulator, and to providesufficient strength at the field joint, for the transfer of the pipeweight through spacers to the casing. Field joints of this general typeare well known in the art.

The pipe/jacket lengths are then typically placed on rollers.

The casing size (OD and wall thickness) is selected to keep thesubmerged weight of the completed bundle within a given tolerance thatwill allow for the tow of the assembled pipeline system to theinstallation site.

FIG. 5 illustrates the assembly with the pipe 10A/jacket 11A locatedinside the casing 12A.

A casing length is fabricated, of the same length as the assembled pipe.The ID of the casing 12A is such as to allow insertion of the thermalinsulated pipe and jacket with spacers 20. The casing is placed insequence with the pipe and jacket.

The casing may be coated with a corrosion coating (such as fusion bondedepoxy), and an abrasion coat may be applied, suitably to the bottom halfof the casing only.

A cable from a winch is inserted into the full length of the casing, andattached to the pipe and jacket.

Bundling Operation

The pipe or pipes with their jacket or jackets are assembled in acluster and pulled into the casing. This operation is called thebundling operation.

As the pipe(s) are pulled into the casing line, spacers 20 are clampedaround the pipe(s) at the field joints, to position the pipe(s) relativeto the casing. The spacers 20 may be provided with wheels or runners toreduce the pulling loads induced by friction, and to prevent damage tothe HDPE jacket at the mid span sag between spacer supports.

It will be understood that the thermal insulation material could beinjected into the jackets after the jackets/pipes have been locatedinside the casing.

Typically, air tight bulkheads are welded to the beginning and end ofthe bundle after the pipe(s) have been inserted into the casing. Endstructures housing valves and connections are typically also attached toboth ends of the bundle.

Whilst FIGS. 3 to 5 illustrate an assembly with a single inner pipe andjacket, as in FIG. 1, it will be understood that similar techniqueswould be employed for pipe bundles such as that shown in FIG. 2. Thiswould require a different type of spacer between the jackets and thecasing, as is well known in the art. In all cases, the spacers betweenthe inner pipe and the jacket and between the jacket and the casing arepreferably of a type which allow gases and liquids to pass therethrough.

Bundle Move-Over, Launch and Tow to Site

The completed bundle (or pipeline assembly) is moved over into the surfwith shore equipment.

Nitrogen under pressure is injected into the casing. The nitrogenpressure is such as to offset the hydrostatic pressure at theinstallation site of the bundle and prevent buckling.

The casing size (OD and wall thickness) is selected to keep thesubmerged weight of the completed bundle within a given tolerance, whichwill allow for the tow of the bundle system to the installation site,with the available towing vessel(s).

A towing line from the towing vessel is transferred to shore andattached to the front end of the bundle. The bundle (or pipelineassembly) is launched and bottom towed to the final installation site.

Specific features and advantages of the invention are set out below:

1. A pipeline system as described, consisting of a steel pipeline(s)surrounded with thermal insulation enclosed in a pipe jacket forapplication in a pressurised fluid environment. The steel pipe, thermalinsulation and jacket are inserted in a steel casing pipe, where thecasing annulus is pressurised to enhance collapse resistance fromexternal pressure in deep water.

2. The use of HDPE jackets or similar thermoplastic or metal jacket inthe fabrication of thermal insulated steel pipelines.

3. The use of HDPE jackets as thermal insulation.

4. The use of a HDPE jacket to serve as a mould for the injection ofthermal insulation material such as open cell polyurethane thermalinsulation.

5. The selection of the thermal insulation material and the thickness ofthe insulation to control the heat transfer coefficient of the system.

6. The fabrication and installation of continuous thermal insulatedpipeline(s).

7. The fabrication of thermal insulated short pipe joints.

Improvements and modifications may be incorporated without departingfrom the scope of the invention.

1. A thermally insulated pipeline assembly comprising an inner pipe, ajacket surrounding and spaced from the inner pipe so as to define agenerally annular space between the inner pipe and the jacket, and anouter casing surrounding and spaced from the jacket, the space betweenthe jacket and the casing being capable of containing gas under pressureand in which the jacket includes apertures to allow the gas underpressure access to the generally annular insulated space between thepipe and the jacket said space being substantially filled withinsulating material.
 2. An assembly as claimed in claim 1, in which thejacket is of high density polyethylene or of other thermoplastic ormetal.
 3. An assembly as claimed in claim 1, wherein the inner pipe issupported within said jacket by first spacer elements disposed along thelength of the inner pipe.
 4. An assembly as claimed in claim 1, whereinsaid jacket is supported within said casing by further spacer elementsdisposed along the length of the jacket).
 5. An assembly as claimed inclaim 1, in which there are at least two inner pipes surrounded by andspaced from individual jackets, the jackets being surrounded by andspaced from the casing.
 6. An assembly as claimed in claim 1, in whichthe insulating material is open cell foam.
 7. An assembly as claimed inclaim 6, wherein said foam comprises polyurethane foam.
 8. An assemblyas claimed in claim 6, wherein said foam has a density in the range 1 to15 lb/cu ft (16 to 230 Kg/m³).
 9. A method for the production of apipeline assembly, comprising the steps of arranging at least one innerpipe within at least one jacket with a generally annular spacetherebetween, introducing insulating material into the annular spacebetween the at least one inner pipe and the at least one jacket,arranging a casing to surround the at least one jacket with a spacetherebetween and pressurising the interior of the casing, and providingthe at least one jacket with apertures to allow pressurized gas topenetrate the thermally insulating material.
 10. A method as claimed inclaim 9, in which the thermally insulating material is introduced in aliquid state, and is allowed to solidify into an open cell foam withinthe annular space.
 11. A method as claimed in claim 10, in which thejacket is arranged to lie generally horizontally during introduction ofthe insulating material.
 12. A method as claimed in claim 9, in whichthe jacket is arranged to lie at a slight slope during introduction ofthe insulating material, and then arranged to lie horizontally duringcuring of that material.
 13. A method as claimed in claim 9, includingdisposing first spacers along the length of the inner pipe whereby saidinner pipe is supported within said jacket.
 14. A method as claimed inclaim 9, including disposing further spacer (16) elements along thelength of the jacket whereby said jacket is supported within saidcasing.